1. Field of the Invention
The present invention relates to a manufacturing method of a semiconductor device, in particular relates to an exposure method for a semiconductor device
2. Description of the Related Art
In a photolithography process during manufacturing an electronic device such as a semiconductor element, a liquid crystal display element, an image capture element such as a charge coupled device (CCD), a plasma display element and a thin film magnetic head (hereinafter generically referred to as “electronic device”), a pattern of a photomask or a reticle (hereinafter generically referred to as “reticle”) is transferred by a projection exposure device having a projection optical system onto a substrate such as a wafer or a glass plate, a surface of which is coated with a photosensitizing agent such as a photoresist. As the projection exposure device, there is employed a reduced projection exposure device of a step and repeat system (so-called stepper), a scanning projection exposure device of a step and scan system (so-called scanner), or the like.
When the pattern is transferred onto the substrate by the projection exposure device, in order to suppress the occurrence of an exposure failure caused by defocus of the projection optical system, there is a need that an exposure area (an area irradiated with an illumination light) on the substrate is disposed in a range of a focal depth of the best imaging surface of the projection optical system. To achieve this, the best focus position of the projection optical system must be measured with high precision, and a position of the substrate must be controlled in such a manner that the exposure area on the substrate is disposed at the best focus position. With the recent miniaturization of the exposure pattern, a higher focusing precision has been required.
As one of methods of measuring the best focus position of the projection optical system, there is a method in which a measurement mark formed on the reticle, for example, a line and space mark is irradiated with the illumination light, a spatial image (projection image) of the measurement mark formed by the projection optical system is measured by the aid of a spatial image measuring device, and the best focus position is calculated on the basis of the measurement result. Exemplary documents are JP 2006-108305 A and JP 2002-195912 A.
In the measurement of focus and the correction of focus by the spatial image measuring device, the focus correction can be processed at five positions within a shot inside of the wafer, but is performed at less than five positions in the exposure area (shot) protruded from the wafer outer periphery, giving a little information After a calculation, information low in precision which is short in the focus range within the shot and slope data is fed back to a stage drive device.
As a result, in the reduced projection exposure device having a short focal point and a larger numerical aperture (NA), when the miniaturized pattern is transferred and projected, defocus occurs, thereby making it difficult to accurately transfer a faithful pattern of the reticle.
Accordingly, even if the above-mentioned best focus position is measured around the best focus position at the time of previous measurement, there is a case in which the best focus position cannot be detected. In this case, the spatial image is measured at 15 steps in the manner as described above while stepping the measurement pattern in an optical axial direction of the projection optical system at a relatively large interval of about 2 times the step pitch in normal measurement (for example, 0.3 μm). Thus, while a large measurement range (measurement range twice as large as normal measurement) is covered, measurement (called “rough measurement”) for roughly searching for the best focus position is performed. Thereafter, the above-mentioned normal measurement (called “fine measurement”) is performed around the obtained best focus position to detect the best focus position with high precision.